Flat panel displays (hereinafter referred to as “FPDs”) are required to accommodate higher definition, higher display frequencies, and reductions in power consumption, as a result of the spread of TVs and smartphones. With this trend, the TFTs for use in circuits which drive such displays have come to be required to have high-speed responsiveness, i.e., high mobility as semiconductor characteristics.
Amorphous oxide semiconductor thin films (hereinafter referred to as “oxide semiconductor thin films”) are receiving attention nowadays as a semiconductor thin-film material for constituting TFTs. In particular. In—Ga—Zn—O4 (hereinafter referred to as “IGZO”) is being investigated as a promising material. IGZO has higher mobility than amorphous silicon (hereinafter referred to as “a-Si”), which has hitherto been used, and is capable of accommodating higher definition and effective in reducing leakage current, and hence has advantages of contributing to a reduction in the power consumption of FPDs. IGZO is consequently expected to be used in applications such as next-generation displays, which are required to have a larger size and higher resolution and to be driven at a higher speed.
However, there are cases where an oxide semiconductor thin film has electrical defects introduced therein due to a compositional change attributable to the inclusion of multiple components, a structural fluctuation attributable to the amorphousness, etc. An oxide semiconductor thin film, in particular, considerably changes in carrier concentration, which governs the TFT characteristics, because of lattice defects generated in the deposition step or of hydrogen in the film, or changes in electronic state because of a subsequent heat treatment, thereby affecting the quality of the TFT. Thus, there are fluctuations in mobility due to film quality and the threshold voltage (Vth) shifts through imposition of negative-bias stress under light irradiation result in changes in switching characteristics, etc., and there is a problem in that these affect the TFT characteristics. For example, a TFT employing IGZO which has been incorporated into an FPD deteriorates in switching characteristics because of stress due to light to which the TFT is exposed during use or due to a voltage applied thereto during standby. Meanwhile, in FPDs employing OLEDs (organic light emitting diodes), the Vth shifts due to the influence of a positive-direction driving voltage for causing the OLEDs to luminesce. Since the TFT characteristics are attributable to the electronic state of the oxide semiconductor thin film, it is considered that the deterioration in switching characteristics due to stress is also attributable to a change in the electronic state of the oxide semiconductor thin film.
It is therefore important from the standpoint of improving production efficiency that in steps for producing an oxide semiconductor thin film, the electronic state of the oxide semiconductor thin film should be grasped to evaluate the production process for any influence on the electronic state and the results of the evaluation should be fed back to regulate the production conditions and control the quality of the TFT.
As a method for determining threshold voltage change ΔVth (hereinafter often referred to as “threshold shift ΔVth”), which affects switching characteristics, an LNBTS (light negative bias temperature stress) test is adopted, which is an accelerated test for simulating the state of a standby TFT which is receiving a negative gate voltage (negative bias) and is continuously irradiated with stray light from a backlight. Meanwhile, a PBTS (positive bias temperature stress) test is adopted as an accelerated test for simulating the state of a standby TFT to which a positive gate voltage (positive bias) is being applied. The LNBTS test and the PBTS test are for determining a change in threshold voltage through stress imposition. The smaller the threshold shift ΔVth, which is calculated from the results of the test, the better the stress stability and the better the practical switching characteristics. Although the LNBTS test and the PBTS test are commonly used as highly reliable evaluation methods, it is necessary, for carrying out these tests, to actually produce a TFT to which electrodes have been attached and this production requires time and cost. There has hence been a desire for a technique capable of more easily and accurately evaluating stress stability.
Patent Document 1 and Non-Patent Document 1 each disclose a method for qualitatively or quantitatively evaluating the stress stability of an oxide semiconductor thin film by a microwave photoconductivity decay method (μ-PCD technique), as a method for evaluating the stress stability of an oxide semiconductor thin film in a non-contact manner without attaching electrodes thereto.